Is Double-Stud Wall Construction the Path to Efficiency on a Budget?

An owner-builder inherits a foundation and now needs a house

Double-stud walls save energy. Double-stud walls allow lots of room for insulation while minimizing thermal bridging. Will they work on a foundation made for a more conventional house?

UPDATED with an expert opinion from Bruce King

Writing from Glacier, Washington, Karen Bean faces a home-building dilemma that confronts many thousands of people: what's the best way to insulate the walls of her new house on a modest budget?

She has $150,000 to spend on the two-bedroom, two-bathroom house, which she plans to build on a foundation originally intended for a traditional house. Although the concrete-block foundation is well made, it's not necessarily well matched to the double 2x4 walls she's hoping to use.

"Is double wall with blown fiberglass insulation worth the extra cost?" she asks in her Q&A post. "And can it be done on an already laid concrete block foundation? Or is there another equally warm option that I just don't know about?"

Bean's quandary is the subject of this week's Q&A Spotlight.

Suggestions for a truss wall system

Bean helps clear up some confusion over exactly what climate zone Glacier is in; it turns out that the site is relatively cold. Glacier is about 30 miles east of Bellingham, WA, and usually sees about 7,000+ heating degree days per year.

And that, says Robert Riversong, makes it a good candidate for a Riversong Truss house, an energy-efficient design he's been building in New England for many years. The 12-in. thick walls are filled with dense-packed cellulose, not fiberglass.

"I typically build with locally sourced, green, rough-sawn lumber, no exterior sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen.
and horizontal shiplap siding, with the Air-Tight Drywall system as the interior air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both.," Riversong writes. "This makes a highly breathable, highly insulative (R-45) wall with almost no thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel.
, and requires no more lumber than a conventional 2x6, plywood sheathed house. And it's about as green as can be done with conventional materials."

Rob Harrison is using a similar approach on a cabin not far away, in Wauconda, WA. Harrison hopes that his cabin will meet the PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. standard.

His walls will include 15-in. Larsen trusses over structural sheathing on 2x6 studs, all stuffed with dense-packed cellulose. "This yields a wall of R-70," Harrison says. "Roof assembly is R-85, floor over crawl space is R-75. Windows are U-0.13."

Both Harrison and Riversong think straw-bale construction might be another option, although Harrison cautions against it in certain parts of the state.

Plans first, foundation second

James Morgan probably wasn't the only one wondering why Bean is pondering wall systems when the foundation is already in the ground.

"Karen," he says, "I have to ask: how come the foundation is in place before you have decided on what wall system is to go over it? It can't be over-emphasized that good forward planning of your whole-house construction strategy is essential if you a) want to build green and b) do it on a tight budget."

It turns out that there's a good reason for this, even if it makes the project more challenging. As Bean explains the next time her dial-up modem agreed to work, the foundation was "inherited."

"The foundation is in because we inherited it," she says. "It’s a well-made foundation — concrete block, basement, nice crawl space — so I’m going with it. The original house was to be traditionally built. So I’m trying to figure out how to adapt what I have to something that would be more insulated than a traditionally built wall. If it’s not possible, I’ll live with it."

Is rough-sawn lumber a good way to save money?

Riversong typically includes rough-sawn lumber in his houses. In a heavily forested area that supports local saw mills, buying green, rough-sawn material can add up to some significant savings. Manufacturing costs and transportation costs are both lower.

Harrison, who is based in Seattle, isn't so sure. "I would strongly caution you against building with green lumber in western Washington," he says. "I caution against green lumber because the builders with whom I work here (who have been doing super-insulated houses for 30 years, back to before the BPA Super Good Sense days) have had trouble with it here in western Washington," he tells Riversong. "That is, based on their feedback I don't think you can just substitute green lumber in a conventional building assembly, and not have some issues. (Twisting, shrinkage, mold.) I'm sure green lumber works in your complete system, where you are. "

Green, rough-sawn material may be heavy, Riversong says, but it has a lot going for it, even if builders and local code officials don't always embrace it.

"The first principle of sustainability (which should be the basis of what is called 'green' building) is to use locally-sourced materials that are processed as little as possible," he says. "This not only limits transportation costs and embodied energyEnergy that goes into making a product; includes energy required for growth, extraction, and transportation of the raw material as well as manufacture, packaging, and transportation of the finished product. Embodied energy is often used to measure ecological cost. and global warming contribution, but also supports the local economy which sustains community and allows us to live responsibly within the annual output of our local environment."

Once green lumber is incorporated into a frame, it dries quickly and stays straight. Plus, Riversong says, "it cuts like butter" and nails with no resistance.

David Meiland has had similarly positive experiences with locally milled wood.

"We have excellent locally grown fir available here," he says. "The inspectors are familiar with this material and don't even blink, although maybe they're supposed to have you bring a lumber grader on the job to stamp everything. It is a little more expensive than commodity lumber and it's harder to get an order filled and delivered (the sawmills are small and manual here)."

On the issue of costs

Costs are the bugaboo of any building project, especially for owner-builders, and here there is no surefire way to keep them down.

As GBAGreenBuildingAdvisor.com senior editor Martin Holladay points out, "If you are an owner-builder, it's hard for people in other parts of the country to recommend an inexpensive way for you to build your wall. It depends on who is doing the work, whether you are paying for labor, how much time you have, and whether you are willing to scrounge for recycled materials."

But the use of rough-sawn lumber from a local mill is one way of holding down costs, he says, as is the possibility of picking up recycled polyisocyanurate insulation that's been salvaged from commercial roofing jobs. Ask around to see if any recycled foam is locally available, he suggests.

"If you are paying for the labor, it's hard to beat double-stud walls filled with cellulose insulation for a low-cost high-R wall," Holladay adds. "But even that option may still be too costly for your budget."

One way of making a project less expensive is by making it smaller, as Harrison suggests. "Have you considered somehow paring down the program?" he asks. "Let's say, doing one 'rush hour' bathroom instead of two full baths?"

Doing some of the work herself, or getting help from friends and neighbors, is often the best way of holding down costs, Riversong says, along with keeping the geometry of the house as simple as possible.

If Bean is having someone else design and build the house, she should plan on spending about $150 a square foot for something that's well made.

Expert's opinion

Our expert's opinion this week comes from Bruce King, a professional engineer and director of the Ecological Building Network:

Eliminating the plywood skin may be fine in New England, as Robert Riversong does, but unless Karen has otherwise allowed for seismic forces she really should keep it. Being on shaky ground really changes the ball game.

The Larsen trusses are easy: just think of the exterior line of studs as the bearing wall, and add inwards as needed for insulation. It doesn't matter if the interior studs don't align over the foundation, as they aren't taking any load. It's really conventional framing with thick insulated furring.

If you don't want to lose too much interior space, reduce the thickness of the cellulose (or fiberglass), and add a layer of rigid outside the plywood. But first read other articles and blogs on GBA about what can happen when a superinsulated wall doesn't allow enough "bleed" heat to dry the siding; superinsulation done wrong, especially in rainy Washington, can exacerbate moisture risks.

Straw bale construction is a perfectly viable option if you pay attention to keeping the bales dry during construction and thereafter. The plastering may seem like extra work, but it also gives you very very airtight buildings that you could otherwise only achieve with a lot of caulking and taping.

Some climates can easily justify a "double wall"
What is clear is that walls in almost all climates need a thermal break.
Foam is not the ONLY option.
Cross-hatch or a variation on the "Mooney Wall" would be a good starting point.

2.
Jan 10, 2011 9:43 AM ET

John post more info on the "Mooney Wall" by aj builder, Upstate NY Zone 6a

Great blog, I agree with most all of it.

I just read that here in NY we can use rough sawn lumber per our new 2010 code and it is simple. The most we have to do is get a letter from the mill stating it is number 2 grade. No engineer and no stamps.

I also agree that local sawn is better for the planet. But as to price, our local lumber costs more than store purchased stamped kiln dried framing lumber. Not less. Worth paying more to do the right thing and the cost is not too much higher but it is higher.

3.
Jan 10, 2011 1:54 PM ET

Keep it simple by Nick Baxter

Keep it simple. I prefer foam over a 2x6 advanced framed stud wall. By keeping the wall narrower, you are keeping your window and door casings narrower and reducing the amount of lumber needed in the walls themselves and any trim casings. By using 1-1/2" of foam (two layers 3/4" foam) and R-19 fiberglass cavity insulation, your are outperforming the Washington State Energy Code (Zone 1) by about 50% (by providing a thermal break, Zone 2 now requires a thermal break however) and only increasing the cost minimally, about 5% materials cost over using the standard R-21 fiberglass insulation.

If a further increase in R-Value is desired, then start looking at options like adding a Mooney System (horizontal 2x2's on the interior), a third layer of 3/4" foam, or going to a 2x4 double wall. The advantage of foam is that it spans between floors and all the way down to the foundation footing, this provides a true thermal break on the entire envelope of a building.

Performance is in the details, air seal and insulate everything well. I also suggest using energy heels in your roof trusses to get the full depth of roof insulation where the exterior walls meet the roof. (Don't forget, scissor trusses are a great alternative to stick framed vaulted ceilings and offer a great thermal break.)

4.
Jan 10, 2011 3:58 PM ET

Nick Baxter where have you been, I need back up. by aj builder, Upstate NY Zone 6a

Your post is welcome here. You will be shouted down by the foam hating police. Stay your course. The time is coming when you will be able to convert to bio foam sheating. And the time will come when a more natural OSB will be available. I think with the likes of Robert Riversong pushing natural green we all may get to real green in time.

Foam exterior sheathing is "simple" like electricity is 100% efficient for heating - in both cases, only at the end use, but ignoring all the ecological and social "externalities" along the way. And there is nothing "green" about petrochemicals.

A Larsen Truss or Riversong Truss wall also creates a continuous thermal break from eaves to sill, but without resorting to un-green materials.

7.
Jan 12, 2011 4:42 PM ET

Expert? by Riversong

I don't know Bruce King or what his alleged expertise is, but he confuses Larsen Trusses with some kind of interior double wall system.

The Larsen Truss, invented by John Larsen of Alberta Canada, is a light-weight parallel-chord truss that attaches to the outside of a sheathed wall. It has the advantage over typical double-wall systems that it creates a continuous thermal break and exterior insulation cavity. What makes it awkward for new construction is that it creates a secondary insulation cavity.

For that reason, I came up with the Riversong Truss wall, which uses the primary frame as the load-bearing structure and extends an outer truss chord on plywood web members or sawn lumber gussets to carry the siding and windows while creating a continuous un-bridged single insulation cavity that makes dense-packing with cellulose easy and efficient.

Building a secondary wall inward of the load-bearing wall creates all kinds of problems, including the thermal bridging and air-barrier interruption of the floor assemblies, and ambiguous load paths, since the interior wall will pick up floor loads and may not be supported by the foundation.

There are, however, techniques for building a double-stud wall such that the interior wall carries the floor assemblies and the outer wall carries the roof load. Both walls, however, need to have their loads carried down to the foundation. For either this system or my Riversong Truss wall, I prefer a foundation (with insulation) as thick as the wall it supports, but there are ways to use these envelopes on conventional foundations as well.

8.
Jan 12, 2011 5:41 PM ET

Dense-packed cellulose cellulose question by Tom MacCallum

Here in northern Utah, 7000'+, I've seen cellulose filled walls that have settled and left huge voids between the studs. I'm pretty sure I recall one of the inspectors telling me they won't allow it's use anymore in stud walls. So my question is what is dense packed cellulose? How is it made "dense"? Wouldn't a blown-in batt system in the wall thickness you all are recommending be better? Higher R value? Prolly not green though.

9.
Jan 12, 2011 6:05 PM ET

Good point Robert. by Rob Harrison AIA

Yes, for the record Bruce King is describing a "conventional" double stud wall, not a Larsen Truss. Jon Alexander has taken that approach with several of the houses he's built here in western Washington.

I understand the idea of the Riversong Truss now (having looked at Robert's link)--using the inside chord of the truss as the bearing wall. That's the system The Artisan's Group used in their North Passive House in Olympia, WA. Here's a link to a photo:http://www.flickr.com/photos/robharrison/5091546884/in/set-7215762506114...
I can see the advantage, especially in sequencing.

I also prefer to use petroleum-based foam only where necessary. True Nick, there will be alternatives at some point, but I'm not seeing them right yet.

In our proposed Larsen Truss + stud wall system, the inner advanced framed 2x wall bears directly on the foundation. Structural sheathing on the outside of the 2x wall allows conventional solutions to seismic resistance--anchor bolts, straps, etc.--and looks familiar to building inspectors. This would work fine for Karen's existing foundation as well. The Larsen trusses hang off the outside, only supporting the siding. In a less extreme climate the inner 2x wall could be left free of insulation as an "installation layer," so there would be no secondary insulation cavity.

Great discussion. I always learn when I stop by.

10.
Jan 12, 2011 7:25 PM ET

Foam - perhaps not all morally despicable by Carl Mezoff

Another viewpoint might argue that while, in a perfect world, we should minimize our petroleum consumption or end it altogether, on balance, using petroleum to create (long-lived) thermal insulation is a far better use for it than using that same petroleum as a fuel.

11.
Jan 12, 2011 9:53 PM ET

wall system for inherited foundation by Jock Jacober

You have the best scenario for a SIPs wall construction project. Structural Insulated Panels are available from numerous sources and give a very well insulated and easily sealed envelope. The structure is integrated into the wall system. Because you have the foundation in place you have the advantage of using actual built dimensions to order your SIP package, either for the foundation wall or for a sub-floor.

12.
Jan 12, 2011 10:30 PM ET

Effective designs by Perry

Why not use SIP panels, cost effective, easy to put up on exisiting block foundation, meets or exceeds the required R value, less waste at job site

13.
Jan 12, 2011 10:46 PM ET

SIPS, yes by gred

I agree with Jock. Perfect for Sips panel, thought I was liking the aspect of using local wood in a truss system.
How about a recycled-plastic spacer to join the webs, hammer-on, with a gauging feature to hold the alignment true while using irregular local wood.

14.
Jan 12, 2011 11:14 PM ET

Tom M: I see that no one by John Klingel

Tom M: I see that no one answered your question, so I'll tell you what I know (most of that from reading here; I have not done this yet). Dense packed cellulose is blown into a cavity with a "good blower" (Robert R can recite the specs on it, but he recommends a Force 1 or Force 3 blower). The density should be 3 to 3.5 pcf, whereas loose blown, like in an attic, is about 1.5 pcf. Dense packed cellulose will not settle, as it is already more dense that if loose. I suspect that the poor job you sited was just that; poorly orchestrated. Search here for more info, and there are videos on youtube of cats dense packing. builditsolar.com also has a good article about RR's framing, etc. Good luck, and keep reading here. Lots to learn, for me anyway. j

15.
Jan 12, 2011 11:45 PM ET

Many benefits! by Steven Bauman

I agree that SIPs are the way to go-providing they are of urethane foam-These have significantly higher R-values per inch (6 in wall systems has an R-43+ typically) -virtually no thermal bridging, with much longer span capability-reducing the amount of board footage of lumber required in the frame. Also the wall system can be built right off from the foundation with the floor framing occurring inside the 'box'. This not only removes the energy nosebleed at the foundation, but also significantly strengthens the structure if the roof is built with SIPs at the outer plane-(giving insulated and useable attic spaces) and makes it so it will surpass both earthquake requirements and force 4 hurricane wind loading. Additionally, by using a reduced size wall thickness due to the higher R values, you gain approximately 64 square feet of net useable space (the size of a large bathroom) within the structure of an average 1,000 square foot home (6.4% sq ft increase in same size shell). The shell can easily be sized to the existing foundation and will dry in much faster while having minimal waste on site-particularly if its a pre-cut package. This ALL equates to reduced construction costs of the end product for its exterior size compared with traditional and advanced frame methods, netting MORE interior square footage with reduced operating costs. On the homes we designed and built for Habitat this way-the ongoing savings of operations alone reduced the mortgage by 8 years over the traditional construction methods! SIPs!

16.
Jan 13, 2011 12:53 AM ET

wall thickness by Joseph Ford

Steven, you make a good point by reminding people that thick walls have a cost in terms of space, and space isn't free. Whether viewed in terms of increased building footprint or reduced interior space, relative to a 6" wall, a 12" wall represents at least several thousand dollars in higher indirect costs or reduced value.

17.
Jan 13, 2011 1:36 AM ET

Robert Riversong Right About Larsen Truss by Frank Hanlan

I was at the original presentation by John Larsen to the Solar Energy Society & Robert Riversong is exactly right in saying that that the exterior wall hangs outside the foundation wall. I forget whether it was John Larsen or someone from the audience that pointed out the exterior wall could extend below the floor plate and top of the foundation so as to both seal it and provide insulation down past the floor plate so as to ensure that the main &/or 2nd floor wasn't cold at the outside wall.

On March 10, 2010 Harold Orr made a presentation detailing the retrofit that he did to a 4 plex in Saskatoon, SK to the Solar Energy Society of Alberta (SESA) and showed a Larsen truss wall which he supported with stainless steel supports that were shaped like short hockey sticks except that they had a flat piece instead of a blade (slide 17 of his powerpoints). (I don't know where he found them.) They enabled him to build the nonstructural exterior wall 4 ft on center and hung them from the eaves. This meant it was easier and faster than building the individual trusses and attaching them 16 inches or 24 inches on center.

Harold Orr gave permission to SESA to provide his powerpoint presentation to the public but they have taken it down. I have a copy if someone has a place that I or they can put it. At the end of his presentation Harold also provided a detailed analysis of his projected & actual costs for the complete retrofit including the breakdown on the wall systems.

18.
Jan 13, 2011 3:10 AM ET

SIPS sound great but they aren't fun in Reality by Kevin Dickson, MSME

Some of us that have built homes with SIPs have decided they aren't really worth the extra cost and site hassles. Basically, they are just too ungainly for urban infill redevelopment. (A remote new home has two sustainability strikes against it)

The next issue is that they aren't really at the high end of R values for superinsulated homes anymore. Trust me, you wouldn't want to use 12" roof SIPs for your walls, but 6" SIPs are only R26.
BSC doesn't really promote them, mainly because of cost, which is too high for builders aiming for affordability.

Kevin-I believe you when you say they weren't worth the hassle as from your statement of R value it appears that you were using EPS SIPs which are a low performer with not only enevironmental concerns but also additional structural requirement. SIPs of urethane do not require internal structural members and as such go in place much faster and with less labor. A 6" wall is rated at R43 in the raw with finishing moving up from there depending on choices. Here is a link for our environmentally award winning project in Austin, TX-completed 2002: http://www.housingzone.com/pb/article/extreme-building This project, although built in a Hot climate-used SIP urethane wall panels of R28 (4.5") with R 37 (5.5") roof systems. The overall result for energy demand for the 4 homes (Air conditioning being the primary demand) resulted in average DAILY total energy costs ranging from $1.02/day (1,456 sf-1+ story) to $1.42/day (2,380 sf-2+story) with only quality materials and passive utilized.solar utilized (no geo-thermal, active solar, or wind, etc).This project being 8+ years old-could easily be improved upon with many of the improvements in materials technology-but exemplifies performance standards of urethane SIPs. And before anyone goes off on the cost per sq. ft.. -this was a cutting-edge LUXURY development with high end sustainable finishes and technologies that came in at 77% of the average standard cost for luxury in that marketplace at that time. (We have successfully built affordable housing using the same structural system.)

20.
Jan 13, 2011 10:48 AM ET

Urethane Sips by Kevin Dickson, MSME, P.E.

EPS SIPs don't need "internal structural members". In fact, they can be used for roofs and floors without rafters or joists because they get quite strong as they get thicker.

I still like SIPs, mainly I just usually don't have room for them on my jobsites.

21.
Jan 13, 2011 12:04 PM ET

Double wall by Doug McEvers

Karen,

Thank you for asking the question about building double walls, go for it! In this case, where you have a foundation set for a traditional wall thickness, go with Robert's system or a variation thereof. The inner wall will be the load bearing wall, placed over the foundation below. You will then add the outer wall for a total thickness that will yield R-40 or more. I like the densepack cellulose choice as it is green and will fill the insulation space completely.

If I have a new build, my wall system will have no foam, the double wall has so many advantages. Foam is expensive and hard to attach siding and windows to, plus there is no structural strength. I like fiberboard sheathing but you may need solid sheathing in your area, choose plywood over OSB. If you have 7,000 hdd, R-40 walls should be the minimum, R-60 for the ceiling.

22.
Jan 13, 2011 8:58 PM ET

Edited Jan 13, 2011 9:03 PM ET.

Invasion of the Plastic Mongers by Riversong

Have a discussion of double wall building systems and suddenly the SIPs proponents arrive like an invading army of missionaries. Five true believers in "better living through chemistry" assault us with the misinformation and distortions that are all-too-common in the petrochemical foam industry.

Every reputable comparison of various envelope systems has put rigid foam, and particularly SIPs, at the top of the cost curve. They are the most expensive way to insulate a house, in spite of some potential site time savings (which are far less than advertised because of the amount of cutting, melting, splining, fitting, drilling and sealing that is required - in addition to crane time).

Inflated R-values are still prevalent among foam peddlers, even though FTC rules require the use of aged R-values, which for urethane is about 5.5 to 6.35 per inch.

The long-term structural integrity of two pieces of OSB (the lowest quality building material in common usage) glued together with plastic foam is yet to be determined. What began as a system for fabricating walk-in coolers was morphed into a high-tech system for enclosing human beings in a hermetically-sealed "picnic cooler" box and then hooking them up to another high-tech life-support system.

Less waste? All waste from SIPS cutouts (whether on site or at the factory) become a landfill burden which won't go away for a hundred generations (if homo sapiens lasts that long). All the waste from wood-framed homes can be burnt as a carbon-neutral fuel or composted into nutrients for the next generation of living things.

And the myth of loss of value by diminished interior space continues to be used by those with no better arguments. First, truss walls and double walls can be built outward if an existing foundation limits the usable square footage. But $/SF is nothing more than a convenience of the real estate market. The real value of a house is not in a few more inches of space but in the quality of the floor plan, energy efficiency (which includes embodied energy), occupant comfort and health, and the overall impacts on the environment and the future of humanity and other living creatures.

This is a "green" building site, and there are many discussions happening in the Q&A section about the inappropriateness of petrochemicals of any kind to a truly green world. The various double wall and truss wall systems, filled with cellulose insulation, offer a green alternative to the status quo.

It's time we consign the missionaries of plastic to history and take a step toward a sustainable way of life.

23.
Jan 13, 2011 9:37 PM ET

The wall in the picture by Burke Stoller

I was one of the carpenters that helped build the house in the photograph used for this article, which was the Riverdale Net Zero home, built by Habitat Studio and Workshop in Edmonton, Alberta, Canada. (http://www.riverdalenetzero.ca/Home.html). The framer (pictured closest to camera), Adam Larson revealed some of his cost estimates for framing this double wall, as opposed to a conventional single stud wall. Because it was his first time framing a double wall, Adam estimated that the process took him about an extra 50% in time to frame the walls this way. And because he figures that his exterior walls (the only thing requiring an alteration in his typical framing routine) usually only take about 30% of the time required to build a house (the roof and interior walls making up the rest), then for the overall house, the double wall only added about 15% for the framing labour. My project manager for the house, Peter Amerongen, was trying to convince me that the double-wall system used for the Riverdale Net Zero home (as photographed) used no more lumber than a conventionally framed 2x6 wall on 16" centers. I had a hard time believing him, so I sketched up two identical walls: one double wall as we built it with 2x4's on 2' centers, and one 2x6 on 16" centers, both with a 4x5' window using our details and conventional details to frame the opening. I then calculated the actual board feet of lumber used in each wall, and to my shock, the double wall actually used 1.5% LESS lumber than the typical 2x6 wall! I ran the numbers 3 times, but they were correct. The only additional wood that was used, was to line the window and door openings with OSB, and most of that came from wall sheathing off-cuts.

So, for the Riverdale Net Zero house, we built 16" thick walls, allowing us to achieve a real world- R-56 (modelled using HOT 2000) compared to a typical R-17 or so, as per a typical 2x6 wall. We did this using pretty much the same amount of wood, and only about 15% more labour in the framing stage. To me, 3x the R-value for a couple of extra thousand dollars is a HUGE argument in favour of the double-wall system. Especially since it allows framers and all the other trades to perform their work in virtually the same manner as conventional building, with just a few changes. AND, it doesn't enter the home into the tenuous situation of potentially having two vapour barriers on your wall: on inside, one outside (from exterior foam).

Burke Stoller.

24.
Jan 14, 2011 6:54 AM ET

Prefab Riversong Truss As Part of Home Construction? by Allan Crouse

Great posts, subject relates to home I wish to construct on small budget. I spent 2010 pricing SIPs construction, modular homes, ICF foundations and never felt as though I was getting the best bang for my buck regarding energy efficiency. Like I have read in other blogs on GBA; Energy Star is the beginning, not the end goal. All the builders/manufacturers toted ES as the latest and greatest.

Which brings me to these wonderings...this discussion has pretty much agreed on the R-20-40-60 foundation-walls-ceiling as being the minimum envelope for getting into the superinsulated range of home construction. My question is how can I maximize each component without involving too much/any 'plastic' as Robert mentions? Has anyone ever had the Riversong trusses pre-fabed offsite and erected them as 'hollow SIPS'? Would this even be practical? I am talking of a ranch-style single story home in southwestern PA. I realize high heel trusses and these walls filled with cellulose would address the walls and ceiling but what system will bring the foundation (cellar) to our minimum?

We have two precast concrete wall manufacturers in our area, each using embedded plastic to achieve claimed R-15 and R-30 respectively but both assemblies have the conductive steel studs.

I am just looking at the KISS principle to get a total system that will require a minimum of learning curve for the contractors and get me a good home value. (please, no hay bale-wood stove suggestions...I am leaning more toward minisplit heating with LPG appliances).

Thank you all for the information contained in this blog!

25.
Jan 14, 2011 10:39 AM ET

douible wall by Tad Duby

good comments from a variety of good sources, i agree that the green lumber is great to work with having built homes for over 25 years i hate the kiln dried lumber for framing. Also wanted to point out that the greatest wall assembly won,t be effecitvie without a good air seal. SIPS, ICF, Double walls are not tight just because of the design. SIPS for instantce leak like crazy and have inherant problems in cold climates because of the lack of attention to air sealing the home after trades, durring construction, and sealing the chases they engineer into the system. i have had standard stick frame homes, air sealed well, blower door test much tighter than standard SIPS, ICF homes. Choose a thermal break system (sips have thermal bridging remember that) air seal (caulk foam all penetrations in the exterior evnvelope. Hire someone to provide thermal boundary inspection and insulation inspection ( HERS rater) and blower test in the end. Also I am a celluslose fan, you just get tighyter buildings, it is a greener product, usually a local manfactured product, dries because it is a natural fiber, and i beleive a healthier option. Sorry fiberglass boys, Sips, ICF, double wall, fiberglass, cellulose, all good products, but you have to install them correcty so they work.

You are in PA, what are the Heating Degree Days for your location? I do use foam for insulating foundations, on the exterior is my preference. If you are planning a full basement get at least R-10 under the basement floor, R-20 is better.

27.
Jan 14, 2011 12:23 PM ET

Response to Allan Crouse by Riversong

You could probably get any truss manufacturer to make up some parallel-chord trusses that were engineered for your application, but for a one-storey house it's so easy to build double walls that it may not be cost effective to use trusses (and get that by your inspector). Some have used truss-joists on end as studs, and this makes dense-packing easier because of the isolated stud cavities but requires more drilling for mechanicals.

My favorite basement option at this point for a 12" thick wall is the ThermoMass system, which places 4" of XPS in the center of a poured concrete wall, with non-conductive fiberglass ties 12" oc in all directions tying the two wythes of concrete into a monolithic wall. It's a short learning curve for foundation contractors (and ThermoMass will send a rep to instruct them), but it does require a pump truck to place the concrete.

On our 2,500 sf panelized home we ordered extra interior walls and installed them 2 1/2" inside the exterior walls with off-set studs for an additional $2,400 for the walls and $1,800 for the additional insulation. Energy modeling indicated that improving the wall R-value from R-20 to R-41 will save only $70 per year at current energy prices given the other efficiency strategies also used in the home. We were already at very low energy before the walls, the savings might be more if the total energy package were not so aggressive.

To me, all the critical comments about SIPs are good, but the biggest reason for not using them is overlooked: they are unfriendly to owner-builder low-cost homes. The darned big clunky heavy things need a crane to be placed easily, and that's a huge complication most owner-builders simply can't deal with. It immediately moves them into having to hire labor, not to mention the crane. Stick construction, on the other hand, we can do easily.

30.
Jan 19, 2011 11:50 AM ET

RE: Alan Crouse basement questions by MATTHEW AMANN

First, glad your alive Robert Riversong, I couldn't generally agree with you more. @Alan, for a natural cellulose basement material look into Durisol, which is a wood chip cement slurry based ICF block, and Faswall, which is identical to it but made in Oregon, where as Durisol is made mostly in Canada.

32.
Jan 22, 2011 2:20 AM ET

RE:Robert Riversong basement wall by MATTHEW AMANN

The XPS foam centered in the wall assembly seems a little hypocritical coming from your previous comments. That is half the amount of foam that a standard ICF uses. Why not use Durisol or Faswall with the rockwool inserts for basement construction. It's simple, it's easy, uses less concrete, and I believe would come pretty close to the thermal resistance of the Thermomass assembly.

33.
Jan 25, 2011 1:20 AM ET

40 yrs of SIPs...wall moisture by Dan Boccia

Dan Boccia here from Anchorage, AK enjoying this discussion and seeing a place to contribute. First, there are 2 basic kinds of SIPs - a sandwich system in which rigid insulation is bonded to skins with adhesive, and the urethane-injected system in which the outer skins are secured in a press and urethane is injected. Urethane-injected SIPs have far higher R-values than sandwich SIPs of the same thickness. A lumber frame (2 styles avoid full-width studs and thus thermal bridging is minimized) held in enormous presses is injected with urethane at 2000psi. My experience designing and managing construction of new and renovation projects for small water treatment plants and other commercial structures all over Alaska, some dating to the early 70s, tells me that the long-term performance of urethane-injected panelized buildings demands attention. Many of the buildings built in the early 70s are still in excellent condition, take very little fuel to heat, and are unaffected by moisture problems. The R-value of urethane-injected panels has been tested as R44 for 5 1/2" walls. My experience plus testing by manufacturers of aged wall assemblies makes it clear that the long-term R value does not decay much, if at all, and that this system has a track record of nearly 40 years of service in the worst conditions imaginable....like -50F, 100mph winds, drenching coastal storms, high-humidity process piping inside, etc. The proven high long-term R-value, robust resistance to moisture problems, and long-term durability of this system cannot be dismissed in any discussion of "sustainability". Yes, SIPs can be a hassle for owner-builders or small contractors to erect, but given either a small crane or tele-handler and crews of local village laborers, we've had buildings erected in less than a week. Second, moisture is what can kill a wall system, and there are certain wall systems that have much higher potential for moisture problems in cold climates - cellulose packed wall systems are one of them, and urethane-injected walls are not. There are two recent publications that everyone here should read if they haven't yet:http://www.buildingscience.com/documents/reports/rr-0903-building-americ...
and:http://www.cchrc.org/docs/snapshots/RS_2010-03_Exterior_Insulation.pdf
I hope my post adds facts and value to the discussion. I'm not dead set on SIPs, and see much value in a few other wall systems, but I think urethane-injected SIPs have their place in any discussion of high-performing high R-value systems.

34.
Jan 25, 2012 11:14 AM ET

Robert, are you available for by Stephen Horvath

Robert, are you available for consulting? I have an old house in Pittsburgh that I'd like to do a deep energy retrofit to. I'd like to talk to you about my insulation strategy. Can you let me know how I can contact you?

You're the only contractor that I've found, both online and locally, that aligns so closely with my own values.

35.
Jan 7, 2013 10:48 PM ET

Double Wall Construction by Ian Brown

I built my double wall house, 1294 sq ft, 30 years ago. Living in Winnipeg Manitoba means cold winters (-20 to -25 F) and the need for extra insulation.
I built the house myself using a 2x4 loadbearing outside wall and a 2x3 non-loadbearing inside wall. The connection between the walls is 12 inch, 5/8 plywood at the top and bottom plates. The Building codes in Western Canada require a vapour barrier, which is installed on the back side of the 2x3 wall and covered with fibre board sheathing. This prevents the movement of the 6 mil poly due to stack effect. The roof is a 14 inch high heel truss with R60 cellulose
The insulation, fiberglass, is R20 in the space between the walls, R12 in the 2x4 and R7 in the 2X3. All wiring is run in the 2x3 so there is no penetration of the vapour barrier. The ceiling had VB and drywall installed first and the interior walls were installed after. No ceiling lites or outlets, all done from the basement. Basement floor insulated under concrete with 1 ½ styro, walls with 1 ½ SM on OS and R20 inside
Heating is electric forced air furnace with an HRV and central air. Exterior finsh is stucco on w/l over building paper and fibre board sheathing. I built an “air lock” at the front door which accesses the garage and the front door.
We pay $0.0694 per kW.h for electricity.

I have been using exterior 2x4 strapping with a rigid foam sheathing-mineral wool if you don't like foam. I have done this over various stud wall configurations. Inspired by my pole barn shop, next I want to try the following: eliminate the studs. Set a 4x8 every 6-8 feet, depending on your window/patio door sizes, and strap both interior and exterior with offset 2x4 2' o/c. Of course you will need wind bracing, and another vertical furring if you desire horizontal siding- I prefer B&Batten for low cost. This simplifies a lot of things-drywall install, taping the foam, etc. It goes up fast. It doesn't require unfamiliar trusses or other materials. It also provides a 9" cavity to dense pack cellulose into, with very little thermal bridging. A layer of housewrap, taped foam, and the 9" dense pack provide a triple-redundant air barrier, and depending on the sheathing, a solid R-40 wall. Here in WI, I would consider doing it all with rough sawn local pine for further savings.